B-cell chronic lymphocytic leukemia (CLL), the most common leukemia in the US, is an incurable disease of unknown etiology affecting middle-aged and older individuals. Patients with CLL have variable clinical courses: some live for decades without therapy, whereas others succumb in a few years after diagnosis despite treatment. Our data indicate that CLL is a monoclonal disease of autoreactive B cells with receptors for antigen (BCRs) of very restricted structure. BCR structure relates to disease outcome because patients with BCRs without IgVH somatic mutations (U-CLL) have a much more aggressive course than patients with BCRs with somatic mutations (M-CLL). The principles governing the biology of CLL cells are similar to those of systemic autoimmunity. Autoantigens cull susceptible B cells out of the normal repertoire and into the disease process; this autoantigen selection is probably aided by intermittent stimulation by microbes. Autoantigenic drive does not appear to stop at leukemic transformation, and we believe this factor is the most important in determining the fate of CLL cells and also of the patient. For these reasons, defining the specific immunoreactive epitopes of the autoantigens that drive the leukemic process is critical. In our studies we will focus on autoantigens that emerge during cellular apoptosis; these autoantigens can be of native structure or chemically modified during the apoptotic process. The autoantigenic epitopes as well as mimetic epitopes from phage-display libraries will be delineated, key residues involved in epitope-BCR affinity scrutinized, and then altered to obtain variants with greater or lesser binding constants (Aim 1). Because the consequences of autoantigen binding determine leukemic cell fate, it is pivotal to determine these and establish the parameters that decide cell fate. CLL cells will be stimulated with surrogate antigens and native or mimetic epitopes that vary in form (soluble vs. insoluble), valency (monomeric vs. multimeric), and affinity (low, intermediate, and high) for the BCR. Initials studies will focus on the mechanisms whereby low-affinity BCR engagement leads to CLL cell apoptosis (Aim 2). Since in vivo B lymphocytes receive signals from several sources and through multiple pathways, we will look for synergy or antagonism between BCR-mediated signals and those delivered through intracellular Toll- like receptor 9 (Aim 3). The results of these studies will provide a structural and mechanistic understanding of the relevant autoantigens in this disease and how these autoantigens stimulate CLL cells, ultimately impacting on patient survival. They will lay the groundwork for future pre-clinical studies targeting the BCRs and the antigens bound in this incurable disease.